Tire buffing machines are well known in the tire retreading industry. Conventionally, a tire buffing machine includes both a structure for mounting a used tire and a rasp hub which removes the worn tread from the used tire prior to the retreading process. The used tire is rotated while being held against the rapidly rotating rasp hub whose outer perimeter is provided with plural tire rasp blades. When engaged against the used tire, the rasp blades cut or shear small segments of rubber from the worn tread surface area of the tire. In this way, the tire is “buffed” to remove the unwanted used tread and to provide the tire with an evenly textured surface suitable for retreading.
Tire rasp blades, together with interleaved spacers, are assembled on the periphery of the rasp hub between the hub front and back plates. The back plate is typically provided with plural axially directed support pins for mounting the rasp blades and spacers between the spaced front and back plates. Then, the front plate is placed on the support pins against the assembled blades and spacers and locked into place. Typically, rasp hubs are comprised of four stacked blade sections (wherein each section occupies a 90 degree circumferential segment), five stacked blade sections (wherein each section occupies a 72 degree circumferential segment), or six stacked blade sections (wherein each section occupies a 60 degree segment). Thus, each section of the assembled rasp blades is comprised of alternating blades and spacers arranged side-by-side. Furthermore, each section of blades and spacers are angled, or offset, relative to a plane perpendicular to the axis of rotation of the hub to form a cut angle, so that each blade in rotation has an axial cutting swath greater than the width of the blade. The width of a cut swath depends on the cut angle.
During operation, the toothed blades rotating at high speed come into contact and remove the old tread surface of a revolving tire by the cutting and abrading action of the rasp blades. However, this high-speed frictional interaction causes an increase in temperature of the rasp blades. Prolonged exposure to high temperature causes a weakening of the blades and increases the likelihood of blade failure. Typically, blade failure involves breaking off of its teeth or breaking of the blade body, particularly a portion of the body located adjacent an end of the blade. The likelihood of a substantial increase in the temperature of the rasp hub and, in particular, the blades disposed thereon is increased by the generally closed structure of the rasp hub with the blades and spacers disposed around the outer periphery of the hub in a tight-fitting arrangement.
In order to enhance the material properties of a rasp blade after shaping, it is well known to perform an austenitising process, wherein the blade is heated to a temperature above an austenitisation temperature, then the temperature is lowered very fast (quenching), and finally a tempering process is performed wherein the rasp blade is reheated to a tempering temperature. In known solutions, the austenitising process is performed by spreading rasp blade in a layer in an oven and heating the rasp blade for several minutes ensuring that the austenitisation temperature is reached. This austenitisation temperature depends on the material, but could be 880 degrees Celsius.
Even though the above-described enhancement process is performed, the blades are still being worn quite rapidly, and it is an object of the present invention to further improve the durability of the rasp blade.